1. Field of the Invention
The present invention relates to a method for dry-etching a metal thin film or a chromium-containing half-tone phase-shift film and more specifically to a method and an apparatus for dry-etching such a metal thin film, which is applied to a process for preparing a chromium-containing half-tone phase-shift photomask which is used in fabricating, for instance, a semiconductor device as well as a method for dry-etching a chromium-containing half-tone phase-shift film which is used in a pattern-etching process for forming a fine pattern of a metal thin film such as fine electrode patterns (for a flat panel display (FPD) or the like) and color filters. In addition, the present invention also pertains to a chromium-containing half-tone phase-shift photomask which is provided with a pattern formed using such a dry-etching method and a method for preparing the photomask as well as a semiconductor circuit fabricated using such a photomask and a method for fabricating the semiconductor circuit.
2. Description of the Prior Art
As a photomask blank for chromium-containing half-tone phase-shift photomasks, there have been known, for instance, those having such a structure as shown in FIG. 1, which comprises a glass substrate a formed from, for instance, synthetic quartz glass; a light-shielding film or a half-tone phase-shift film b consisting of a thin film of a metal such as chromium or chromium oxide, formed on the surface of the substrate; and a resist layer c of a light-sensitive/electron-sensitive resin, which is formed on the light-shielding or half-tone phase-shift film. The glass substrate a may serve as a support for patterns and therefore, must have a variety of desired characteristic properties such as high transmittance, high uniformity, defect-free characteristics, resistance to washing and excellent flatness. In addition, the film b may serve as a light-shielding material for patterning or a phase-shift film and therefore, should satisfy the desired requirements for various properties such as etching controllability, uniformity, defect-free characteristics, resistance to washing, low stress and high adhesion to the glass substrate. Moreover, the resist layer c has a role as a film for forming a light-shielding film or a half-tone phase-shift film and accordingly, should have a variety of desired characteristic properties such as high-sensitivity/ high resolution, resistance to etching, uniformity, defect-free characteristics and high adhesion to the light-shielding film or the film for forming a half-tone phase-shift film.
A photomask provided thereon with a fine electric circuit pattern has been prepared by wet-etching or dry-etching a chromium light-shielding or half-tone phase-shift film using a photomask blank having such a structure according to the electron beam patterning process or the laser beam patterning process. An example of such a mask-processing scheme is shown in FIG. 2. In this respect, the mask-processing scheme shown in FIG. 2 relates to a light-shielding film of chromium, but the processing scheme for the half-tone phase-shift mask is almost identical to that depicted on FIG. 2 and further details of the processing scheme for forming a half-tone phase-shift mask are described in Japanese Un-Examined Patent Publication (hereinafter referred to as “J.P. KOKAI”) No. Hei 7-140635, the disclosure of which is hereby incorporated by reference.
In the wet-etching, there have recently been highlighted a limit in the dimension control due to the undercut and a limit in the verticality of the etched cross section, and the dry-etching technique has thus been widely used instead.
The dry-etching methods for preparing a photomask and the dry-etching apparatus for practicing the methods are described in, for instance, J.P. KOKAI No. Hei 6-347996, the disclosure of which is hereby incorporated by reference. In this dry-etching technique, a chromium film is etched using a gas comprising, for instance, chlorine gas to which oxygen gas is added, as a reactive ion etching gas.
Moreover, the dry-etching method for preparing a photomask of a chromium-containing film is disclosed in, for instance, Japanese Patent No. 2,765,065, the disclosure of which is hereby incorporated by reference. This patent discloses, in Examples, that when the chromium-containing film is dry-etched by this dry-etching method while using a resist film of a positive electron beam resist EBR-9 (which is available from Toray Industries, Inc.) as a mask and a mixed gas comprising 160 SCCM of chlorine gas, 40 SCCM of oxygen gas and 160 SCCM of wet air as a dry-etching gas, there is not observed any change in the etching rate of the electron beam resist film, while the etching rate of the chromium-containing film increases and the selective (or etching) ratio against the resist film is improved. As a result, the chromium-containing film can sufficiently be patterned by this dry-etching technique. In this connection, the wet air (160 SCCM) in the mixed etching gas comprises about 128 SCCM of nitrogen gas and about 32 SCCM of oxygen gas corresponding to the component ratio of nitrogen to oxygen in the air which is equal to 4:1.
In addition, the semiconductor circuit has recently become more and more finer and the size of the semiconductor circuit is increasingly reduced from 0.2 μm to 0.15 μm. For instance, in case of a semiconductor circuit fabricated using a conventional photomask, the dimensional error observed for the memory circuit portion is large as compared with that observed for the peripheral circuit portion in the memory circuit which comprises the memory circuit portion and the peripheral circuit portion, while such an error is also large even in the logic circuit and thus these errors may adversely affect the characteristic properties of the resulting circuit. For this reason, there has been desired for the development of a half-tone phase-shift photomask which permits the fabrication of a circuit whose dimensional difference between circuits within a semiconductor chip is as low as possible.
If a chromium or chromium oxide film as a light-shielding film or a half-tone phase-shift film is subjected to dry-etching using a chlorine-containing gas and if a pattern is formed on a plane at an almost uniform density, the film can be chromium-etched at an approximately uniform rate throughout the whole surface and accordingly, the dimensional control within a plane can be achieved to such an extent that the in-plane uniformity 3 σ (3× the variance of (measured line width−averaged line width)) ranges from 20 to 60 nm for the line width ranging from 1 to 2 μm.
However, dense patterns (patterns whose area occupied by a resist is small) and coarse patterns (patterns whose area occupied by a resist is large) often coexist in the plane of a practical photomask and if the dry-etching technique is used for forming such a photomask, the etching rate of a chromium film is high at the densely patterned portion and low at the coarsely patterned portion. As a result, the dimensional difference within a plane reaches up to about 100 nm for a designed line width ranging from 1 to 2 μm. A photomask having such a large dimensional difference within the plane cannot be used for the fabrication of, for instance, circuits having a higher integration density such as memory circuits, logic circuits and LSI circuits.
The Japanese Patent No. 2,765,065 described above does not relate to the solution of the foregoing problems, but relates to the improvement of the selective ratio of a chromium film to a resist film.